Touchstones are pieces of fine-grained black stone which are used for assaying gold by comparing the color of streaks made on the surface by alloys of known composition with the color of the streak from an object of unknown composition. The technique has been in use since at least the 6th century BC. The petrographic aspects of the identification of touchstones are reviewed, and 42 historical touchstones or museum specimens of similar-looking stone are described, including the results of petrographic examination. Touchstones usually consist of sedimentary rock types, of which the most frequently occurring ones are tuffs, cherts, and siltstones. See also AATA 20-2183 for another reference to touchstones.